Abstract
A 2-year study with 157 Angus bulls from two consecutive calf crops examined the effects of dietary protein level provided early and late in a 140-day feeding period. Bulls were offered a corn-corn silage finishing diet supplemented with four levels of protein for the first 70 days and with four different levels for the last 70 days. The early supplementation levels, .23, .45, .68 and .91 kg of soybean meal (SBM)/head daily resulted in dietary crude protein (CP) values of 9.6, 10.7, 12.5 and 13.9%, respectively, on a dry matter basis, and average total daily protein consumptions of .59, .67, .83 and .93 kilograms. The late supplementation levels, .14, .36, .59 and .82 kg of SBM/head daily, increased dietary CP to 8.8, 10.0, 11.3 and 12.7% CP, respectively, on a dry matter basis, and provided average total daily protein consumptions of .66, .78, .89 and .96 kg, respectively. Bulls fed .91 kg SBM (13.9% CP) had a higher rate of gain (P<.05) than those fed lower levels during the first 28 days of the trial. In the first 70 days, bulls fed .68 (12.5% CP) and .91 kg of SBM did not differ in gain, but both groups gained more rapidly than bulls fed lower SBM levels. Dry matter conversion to gain had a negative linear (P<.05) relationship with early level of protein supplementation. Bulls fed the .59 and .82 kg levels of SBM gained at similar rates and more rapidly (P<.05) than those fed the lower levels during the last 70 days of the trial. Most of the carcass traits of bulls were not affected (P>.10) by early and late protein levels. These data would indicate that phase feeding of protein may enhance overall performance and feed efficiency of feedlot bulls. When bulls weigh between 200 and 300 kg and gain near 1.50 kg/day, the optimum practical protein level appears to be near 12.5% CP (.83 kg protein/day) in the diet. Diets containing near 11.3% CP (.89 kg protein/day) seem to be optimal for bulls weighing 300 to 400 kg and gaining near 1.20 kg/day. Copyright © 1981. American Society of Animal Science. Copyright 1981 by American Society of Animal Science.